Stellar Spectroscopy, Stellar Evolution, Nucleosynthesis

Research Description

Chemical abundances in nearby stars hold the answers to many
of the most fundamental questions about the universe.

The primordial abundance of the element lithium (Li) is an
important diagnostic of early element formation in the Big Bang
and helps to determine the density and fate of the universe. We
are making observations of faint stars in very old globular
clusters to determine the definitive value for primordial Li.

The enrichment of various chemical elements with time in the
Galaxy reveals the history of massive star formation and the
early production of supernovae. In particular, we can trace the
increase of beryllium (Be) and oxygen (O) over time, since their
production depends on the products of massive stars and
supernovae. We are probing both the oldest stars and those of the
intermediate ages in more detail to develop a greater
understanding of the evolution of the Galaxy. Furthermore, the
study of Be and O in star clusters of the galactic disk helps us
to discern the evolution and mixing in the disk component of our
Galaxy.

Although the composition of stars like the Sun is very similar
to that of the Sun, the abundances of the elements Li and Be show
interesting differences among solar-type stars. This
nonuniformity makes it possible to effectively examine the
insides of stars through their Li and Be content. Several models
have been proposed that cause mixing in the stellar interiors.
Mixing of matter caused by rotation can result in depletion of
surface Li and Be, but Li is depleted much more than Be; we are
studying both elements together. The understanding of the
processes that deplete Li and Be is directly connected to
evaluation of the amount of Li (and Be) that formed during the
Big Bang.

Figure caption: Lithium abundances for seven stars in the old, very metal-poor globular
cluster, M 92. The stars with the highest abundance of Li are above the halo
field star plateau value of 2.25. There are Li-poor stars also. The
dispersion in Li - in otherwise identical stars - points to a mixing mechanism
inside the star that circulates the Li to hotter temperatures where it is
destroyed. The only known parameter which could differ from star to star is
the initial angular momentum. All the stars are slow rotators now, but those
with the highest initial angular momentum would have spun down the most and
thus have the lowest Li at present. The highest Li abundance gives a lower
limit to the primordial Li abundance.